Trinkaus was intrigued by the complicated process of gastrulation, and wanted to know how cells rearrange so that the mesoderm comes to underlie the ectoderm during epiboly and how these cell movements lead to normal inductions for later organogenesis. Then, Trinkaus came across a paper on the process of epiboly in the development of Danio rerio, the zebrafish, by Warren H. Lewis at the Wistar Institute in Philadelphia. In this paper, Lewis proposed a mechanism for epiboly by which the driving force behind the process was not found in the blastoderm (the layer of cells, called blastomeres, that forms during cleavage), but rather in the surface layer of the yolk cell (Trinkaus 2003; Kimmel & Weston 2003). In his proposed model, Lewis assumed that the yolk cytoplasmic layer (YCL) contracts and pulls on the blastoderm. Lewis’s model gave Trinkaus a preliminary hypothesis to begin formulating research questions and designing experiments. Trinkaus, thus, settled on the problem of the mechanism of epiboly as a basis for the research program he would develop at Yale.